Optical network systems are provided with a plurality of optical nodes along their transmission lines, such as add modules, drop modules, and amplifiers, enabling higher capacities and longer distances by transmitting wavelength division multiplexed light. In such wavelength division multiplexing (WDM) optical transmission systems, polarization mode dispersion (PMD) is produced from the differences in the transmission speeds for the two polarization modes of an optical signal propagated inside the optical fiber. This PMD then influences the transmission characteristics. The influence of these PMD characteristics becomes more apparent as the per-wavelength transmission speed rises. In ultra high speed transmission system operating at 40 Gbps and 100 Gbps, for example, PMD characteristics become a significant factor causing degradation of transmission characteristics, and cannot be ignored. PMD characteristics are dependent on the performance of the optical fiber laid down as transmission lines. Particularly, older optical fiber that was laid down in the past may include some optical fiber with extremely poor PMD characteristics, which may in some cases exert fatal influence on the transmission characteristics.
PMD characteristics also randomly change as a result of various factors, such as external environment temperatures, curvature in the optical fiber due to external pressure, and shock forces exerted on the optical fiber. The degree of change also varies considerably: from slow change over the course of a year caused by external temperatures, to a sudden difference in speed by several tens of kilohertz as a result of impact shock. Given such variation, it is not easy to ascertain where and to what degree such variation will occur along a transmission line. In this way, PMD characteristics lead to inflated equipment costs when there is variation in the transmission characteristics (particularly when there are degraded characteristics) of an optical transmission system that has been adapted for higher speeds or longer distances. This in turn increases expenditure for maintenance and administration after the network is put into operation. For this reason, the measurement and management of PMD in optical transmission systems is crucial and in demand.
PMD measurement may be conducted using commonly sold measuring instruments that make use of analog optical components, such as optical fiber gratings. Measuring methods also encompass a variety of different techniques (see, for example, Japanese Unexamined Patent Application Publication No. 2008-209188). Also, in recent years, research and development has been pursued in technology that compensates for PMD produced along a transmission line by means of digital signal processing executed at a receiver. A method has been proposed wherein the filter coefficients obtained in the process of such compensation algorithms are then used to compute the differential group delay (DGD) of the PMD from the transmitting end to the receiving end (see, for example, F. N. Hauske et al, “Optical Performance Monitoring from FIR Filter Coefficients in Coherent Receivers”, OFC/NFOEC 2008, pp. 1-3, February 2008).
With measuring instruments of the related art, or when measuring PMD by using digital signal processing at a receiver as described above, problems like the following occur when maintaining and operating a network in actual practice. With methods that use measuring instruments, it is difficult to measure PMD without affecting signals that are in operation. As a result, measurements are taken during a limited period of time after laying down the optical fiber but before actual operation, which means that long-term variation during network operation cannot be confirmed. In this way, since PMD is measured only during a limited, short period of time, external factors such as environmental variations are left unaccounted for, and the measurements cannot be used as sufficient information for ascertaining network conditions. Moreover, equipment costs are high, which increases maintenance and operation expenses.
Meanwhile, with methods involving digital signal processing at a receiver, the aggregate PMD is measured along the entire line from the transmitting end to the receiving end. FIG. 18 is a network layout diagram illustrating the configuration of PMD measurement using a receiver of the related art. Along the transmission line 2000 there are disposed optical nodes 2001, which may be relays, optical add/drop multiplexers (OADMs), or similar components. Optical signals output from a plurality of transmitters 2010 are transmitted on a plurality of signal paths for each WDM wavelength, and received by receivers 2020 of compatible wavelength. In the example illustrated in FIG. 18, PMD may be measured along the signal paths a1, a2, and a3 at the receivers 2020a and 2020b, respectively. Herein, the signal path a1 is the signal path along which an optical signal of specified wavelength is transmitted from the transmitter 2010a and received at the receiver 2020a. The signal path a2 is the signal path along which an optical signal of specified wavelength is transmitted from the transmitter 2010b and received at the receiver 2020b. The signal path a3 is the signal path along which an optical signal of specified wavelength is transmitted from the transmitter 2010a and received at the receiver 2020b. 
However, PMD measurement of the signal paths a1, a2, and a3 involves measurement of paths that all pass through a plurality of optical nodes 2001, and PMD measurement of the spans (i.e., transmission line segments) a11, a12, a13, a14, and a15 existing between the individual optical nodes 2001 is not possible. In this respect, the inability to measure PMD in terms of individual spans means that the spans with poor PMD characteristics cannot be identified. Furthermore, since measurements are taken using signal light itself, the PMD characteristics on wavelengths that do not exist in signal light cannot be measured. Moreover, the digital signal processing at the receivers 2020 may only be used to measure the differential group delay (DGD), and long-term PMD characteristics (i.e., the mean PMD) cannot be measured. As a result, the PMD characteristics of optical fiber cannot be accurately obtained.
PMD measurement according to the above related art has the following specific disadvantages, which greatly increase administrative and operating costs as a result.                1. Spans with poor PMD characteristics cannot be identified, and much time and effort is involved in identification work, such as blocking optical signals in operation to take measurements. Furthermore, PMD characteristics cannot be accurately ascertained before signal connections are made, which necessitates route switching and other fault handling techniques after signal connections have been made.        2. When switching the signal path line to a redundant line, the PMD characteristics of the switching line cannot be ascertained, and thus an error might occur after switching.        3. Since gradual variation in PMD characteristics with respect to environmental temperature changes or changes over time cannot be fully ascertained prior to operation, there is a possibility that errors due to environmental temperature changes may occur after the network is put into operation.        
One object of the technology disclosed herein is to enable easy, per-span measurement of PMD characteristics at low cost, and without affecting optical signals in operation.